Colour picker Texture paint pie menu
Panels and Menus use the same methods to display properties and operators, the main difference is that a pie menu will show properties floating by themselves unless you create a box as a background for them.

To make your own pie menu start with the pie menu template that is included with blender.

To show a colour swatch that will pop-up a colour picker when clicked you use layout.prop() to display the colour property. To show the colour picker itself you can use layout.template_color_picker().

An example pie menu that shows both -



class ColourPie(bpy.types.Menu):
    bl_label = 'Set Colours'
    bl_idname = 'OBJECT_MT_colour_pie'

    def draw(self, context):
        ts = context.tool_settings
        ups = ts.unified_paint_settings
        ptr = ups if ups.use_unified_color else ts.image_paint.brush

        pie = self.layout.menu_pie()
        row = pie.row()
        # add a colour swatch that can popup a colour picker
        row.prop(ptr, 'color')
        box = pie.box()
        #show the colour picker directly
        box.template_color_picker(ptr, 'color', value_slider=True)